Legal heirs fradulently took dads signatures to sell grandfathers land

My grand father was a business man and died, he alone purchased a farm land in the year of 1970's around 6 acres. My father have one elder brother ( who is the cheater ) and one elder sister. my grand mother is alive now. Now the problem is my uncle, ant, grandmother are cheated my father forcing him to taken signature from him in documents and trying to sell the property and giving very less amount to my father. We are quite uneducated family and i am the only guy who is first educated, can any one kindly guide us what we need to do for this situation ? My father is innocent and my relative used that for selling the property and make money out of it by cheating my father. Let me know, Sushanth